A mutation in the heptad repeat 2 domain of MFN2 in a large CMT2A family

Dominant mutations in MFN2 cause a range of phenotypes, including severe, early‐onset axonal neuropathy, “classical CMT2,” and late‐onset axonal neuropathies. We report a large family with an axonal polyneuropathy, with clinical onset in the 20s, followed by slow progression.     

Charcot-Marie-Tooth disease

Charcot-Marie-Tooth (CMT) disease is the commonest inherited neuromuscular disorder affecting at least 1 in 2,500. Over the last two decades, there have been rapid advances in understanding the molecular basis for many forms of CMT with more than 30 causative genes now described. This has made obtaining an accurate genetic diagnosis possible but at times challenging for clinicians. This review aims to provide a simple, pragmatic approach to diagnosing CMT from a clinician's perspective.     

Neurophysiologic abnormalities in children with Charcot-Marie-Tooth disease type 1A

Although Charcot-Marie-Tooth disease type 1A (CMT1A) initially manifests in the first decade, there are no large studies describing its neurophysiologic features in childhood. We report neurophysiologic findings in 80 children aged 2-16 years with CMT1A who underwent median motor and sensory nerve conduction studies. Neurophysiologic abnormalities were present in all children. Median motor nerve conduction velocity was invariably less than 33 m/s (mean 18.7 m/s, range 9.0-32.9 m/s), with conduction velocities significantly slower in children aged 7-16 years compared with children aged 6 years and below. All children had prolonged distal motor latencies (mean 7.3 ms, range 4.0-12.3 ms). The compound muscle action potential (CMAP) amplitude was reduced from an early age (mean 7.1 mV, range 2.1-13.5 mV), and its normal increase with age was attenuated. Median sensory responses were present in only seven children, all aged less than 9 years and with slowed sensory conduction. Neurophysiologic abnormalities are present in all children with CMT1A from the age of 2 years. Motor conduction slowing progresses through the first 6 years of life and thereafter remains stable. CMAP amplitude is reduced from an early age, and the normal physiologic increase with age is attenuated. Median sensory responses may be recorded in younger children, and their presence does not exclude the diagnosis of CMT1A.     

Two novel missense mutations in FGD4/FRABIN cause Charcot-Marie-Tooth type 4H (CMT4H)

By sequencing of the FGD4 coding sequence in a cohort of 101 patients affected by autosomal recessive demyelinating Charcot-Marie-Tooth disease (CMT), we have identified two novel missense mutations in FGD4 in two patients from consanguineous descent: p.Arg442His in an Algerian patient and p.Met566Ile in a Lebanese girl. The patients present early onset, slowly progressive CMT, with drastic reduction of nerve conduction velocities. These mutations are the second and third missense mutations characterized in FGD4. They are likely to lead to conformational changes in the PH1 and FYVE domains.     

Screening for SH3TC2 gene mutations in a series of demyelinating recessive Charcot‐Marie‐Tooth disease (CMT4)

Charcot‐Marie‐Tooth disease type 4C (CMT4C) is an autosomal recessive (AR) demyelinating neuropathy associated to SH3TC2 mutations, characterized by early onset, spine deformities, and cranial nerve involvement. We screened 43 CMT4 patients (36 index cases) with AR inheritance, demyelinating nerve conductions, and negative testing for PMP22 duplication, GJB1 and MPZ mutations, for SH3TC2 mutations. Twelve patients (11 index cases) had CMT4C as they carried homozygous or compound heterozygous mutations in SH3TC2. We found six mutations: three nonsense (p.R1109*, p.R954*, p.Q892*), one splice site (c.805+2T>C), one synonymous variant (p.K93K) predicting altered splicing, and one frameshift (p.F491Lfs*32) mutation. The splice site and the frameshift mutations are novel. Mean onset age was 7 years (range: 1–14). Neuropathy was moderate‐to‐severe. Scoliosis was present in 11 patients (severe in 4), and cranial nerve deficits in 9 (hearing loss in 7). Scoliosis and cranial nerve involvement are frequent features of this CMT4 subtype, and their presence should prompt the clinician to look for SH3TC2 gene mutations. In our series of undiagnosed CMT4 patients, SH3TC2 mutation frequency is 30%, confirming that CMT4C may be the most common AR‐CMT type.     

Phenotypical features of the p.R120W mutation in the GDAP1 gene causing autosomal dominant Charcot-Marie-Tooth disease

Mutations in the ganglioside-induced-differentiation-associated protein 1 gene (GDAP1) can cause Charcot-Marie-Tooth (CMT) disease with demyelinating (CMT4A) or axonal forms (CMT2K and ARCMT2K). Most of these mutations present a recessive inheritance, but few autosomal dominant GDAP1 mutations have also been reported. We performed a GDAP1 gene screening in a clinically well-characterized series of 81 index cases with axonal CMT neuropathy, identifying 17 patients belonging to 4 unrelated families in whom the heterozygous p.R120W was found to be the only disease-causing mutation. The main objective was to fully characterize the neuropathy caused by this mutation. The clinical picture included a mild-moderate phenotype with onset around adolescence, but great variability. Consistently, ankle dorsiflexion and plantar flexion were impaired to a similar degree. Nerve conduction studies revealed an axonal neuropathy. Muscle magnetic resonance imaging studies demonstrated selective involvement of intrinsic foot muscles in all patients and a uniform pattern of fatty infiltration in the calf, with distal and superficial posterior predominance. Pathological abnormalities included depletion of myelinated fibers, regenerative clusters and features of axonal degeneration with mitochondrial aggregates. Our findings highlight the relevance of dominantly transmitted p.R120W GDAP1 gene mutations which can cause an axonal CMT with a wide clinical profile.     

Coexistence of Charcot‐Marie‐Tooth disease type 1A and anti‐MAG neuropathy

At age 35, a man with a genetic diagnosis of Charcot‐Marie‐Tooth disease type 1A (CMT1A) but no family history of neuropathy and no clinical symptoms developed rapidly progressive loss of balance, distal limb numbness, loss of manual dexterity, and hand tremor. Five years later, he walked with support and had mild pes cavus, marked sensory ataxia, severe leg and hand weakness, absent deep tendon reflexes (DTRs), severe sensory loss, and hand tremor. He had dramatically reduced motor nerve conduction velocity (MNCV), strikingly prolonged motor distal latencies, absent sensory action potentials and lower limb compound muscle action potentials. CMT1A duplication was reconfirmed but the dramatic change in his clinical course suggested a superimposed acquired neuropathy. An IgM‐kappa monoclonal gammopathy of uncertain significance (MGUS) with high titer anti‐myelin associated glycoprotein (anti‐MAG) activity was found. Nerve biopsy showed severe loss of myelinated fibers with onion bulbs, no evidence of uncompacted myelin, and few IgM deposits. Rituximab was given and he improved. It is very likely that this is a chance association of two rare and slowly progressive neuropathies; rapidly worsening course may have been due to a “double hit”. Interestingly, there are reports of possible superimposition of dysimmune neuropathies on hereditary ones, and the influence of the immune system on inherited neuropathies is matter for debate.     

Whole‐exome sequencing reveals a novel missense mutation in the MARS gene related to a rare Charcot‐Marie‐Tooth neuropathy type 2U

Charcot‐Marie‐Tooth (CMT) is a heterogeneous group of progressive disorders, characterized by chronic motor and sensory polyneuropathy. This hereditary disorder is related to numerous genes and varying inheritance patterns. Thus, many patients do not reach a final genetic diagnosis. We describe a 13‐year‐old girl presenting with progressive bilateral leg weakness and gait instability. Extensive laboratory studies and spinal magnetic resonance imaging scan were normal. Nerve conduction studies revealed severe lower limb peripheral neuropathy with prominent demyelinative component. Following presumptive diagnosis of chronic inflammatory demyelinating polyneuropathy, the patient received treatment with steroids and intravenous immunoglobulins courses for several months, with no apparent improvement. Whole‐exome sequencing revealed a novel heterozygous c.2209C>T (p.Arg737Trp) mutation in the MARS gene (OMIM 156560). This gene has recently been related to CMT type 2U. In‐silico prediction programs classified this mutation as a probable cause for protein malfunction. Allele frequency data reported this variant in 0.003% of representative Caucasian population. Family segregation analysis study revealed that the patient had inherited the variant from her 60‐years old mother, reported as healthy. Neurologic examination of the mother demonstrated decreased tendon reflexes, while nerve conduction studies were consistent with demyelinative and axonal sensory‐motor polyneuropathy. Our report highlights the importance of next‐generation sequencing approach to facilitate the proper molecular diagnosis of highly heterogeneous neurologic disorders. Amongst other numerous benefits, this approach might prevent unnecessary diagnostic testing and potentially harmful medical treatment.     

CMT4D (NDRG1 mutation): genotype–phenotype correlations

Charcot‐Marie‐Tooth (CMT) disease is a heterogeneous condition with a large number of clinical, electrophysiological and pathological phenotypes. More than 40 genes are involved. We report a child of gypsy origin with an autosomal recessive demyelinating phenotype. Clinical data, familial history, and electrophysiological studies were in favor of a CMT4 sub‐type. The characteristic N‐Myc downstream‐regulated gene 1 (NDRG1) mutation responsible for this CMT4D phenotype was confirmed: p.R148X. The exact molecular function of the NDRG1 protein has yet to be elucidated.     

Charcot-Marie-Tooth disease type 1B: marked phenotypic variation of the Ser78Leu mutation in five Italian families

Objectives – To describe clinical, electrophysiological and genetic data of five unrelated Sicilian pedigrees harbouring a heterozygous Ser78Leu mutation in the myelin protein zero (MPZ) extracellular domain. Materials and methods – Clinical, electrophysiological and genetic findings of 16 patients were reported. Polymorphic markers flanking the coding sequence of MPZ gene were also analysed. Results – A wide range of age at onset was observed in families 1 and 3, with a clinical heterogeneity, in terms of severity of the disease, within the same family (families 1 and 3), and among families. A markedly unsteady gait was a distinctive feature of many members of family 1. All patients in family 2 complained of severe cramps and painful paresthesia. Molecular genetic analysis showed that all affected subjects shared a common haplotype at three microsatellite loci D1S2858, D1S2624 and D1S484. Conclusions – Our study provides further evidence that phenotypic features of MPZ mutations can vary within and among different families. High frequency of Ser78Leu mutation in Sicily as well as the results of haplotype analyses suggest that the mutation may have been inherited from a common ancestor.     

A recurrent GARS mutation causes distal hereditary motor neuropathy

We found a p.Gly327Arg mutation in GARS in two unrelated women, both of whom had a similar phenotype ‐ motor weakness that began in late childhood, distal weakness in the arms and legs, a motor greater than sensory neuropathy with slowing of motor and not sensory conduction velocities. A de novo mutation was proven in one patient and suspected in the other. The p.Gly327Arg GARS variant did not support yeast growth in a complementation assay, showing that this variant severely impairs protein function. Thus, the p.Gly327Arg GARS mutation causes a distal motor neuropathy.     

Anterior tibialis cmap amplitude correlations with impairment in CMT1A

Introduction: CMT1A is the most common form of Charcot‐Marie‐Tooth disease (CMT), a slowly progressive neuropathy in which impairment is length dependent. Fibular nerve conduction studies to the anterior tibialis muscle (AT) may serve as a physiological marker of disease progression in patients with CMT1A. The objective of this study is to determine whether the AT compound muscle action potential (CMAP) amplitude correlates with impairment in patients with CMT1A. Methods: We correlated AT CMAP amplitudes and impairment measured by the CMT Neuropathy Score (CMTNS) in a cross‐section of 121 patients with CMT1A and a subset of 27 patients with longitudinal data. Results: AT CMAP amplitudes correlated with impairment as measured by the CMTNS in cross sectional analysis. Longitudinal changes in the AT CMAP showed a strong inverse correlation with leg strength but not other components of the CMTNS. Conclusions: AT CMAP amplitude may serve as a useful outcome measure for physiological changes in natural history studies and clinical trials for patients with CMT1A. Muscle Nerve, 2013     

Unique distributions of the gap junction proteins connexin29, connexin32, and connexin47 in oligodendrocytes

Oligodendrocytes of adult rodents express three different connexins: connexin29 (Cx29), Cx32, and Cx47. In this study, we show that Cx29 is localized to the inner membrane of small myelin sheaths, whereas Cx32 is localized on the outer membrane of large myelin sheaths; Cx29 does not colocalize with Cx32 in gap junction plaques. All oligodendrocytes appear to express Cx47, which is largely restricted to their perikarya. Cx32 and Cx47 are colocalized in many gap junction plaques on oligodendrocyte somata, particularly in gray matter. Cx45 is detected in the cerebral vasculature, but not in oligodendrocytes or myelin sheaths. This diversity of connexins in oligodendrocytes (in different populations of cells and in different subcellular compartments) likely reflects functional differences between these connexins and perhaps the oligodendrocytes themselves.     

Psychometrics evaluation of Charcot‐Marie‐Tooth Neuropathy Score (CMTNSv2) second version,using Rasch analysis

Charcot‐Marie‐Tooth Neuropathy Score second version (CMTNSv2) is a validated clinical outcome measure developed for use in clinical trials to monitor disease impairment and progression in affected CMT patients. Currently, all items of CMTNSv2 have identical contribution to the total score. We used Rasch analysis to further explore psychometric properties of CMTNSv2, and in particular, category response functioning, and their weight on the overall disease progression. Weighted category responses represent a more accurate estimate of actual values measuring disease severity and therefore could potentially be used in improving the current version. Copyright © 2014 John Wiley & Sons, Ltd.     

Phenotype expression in women with CMT1X

Charcot-Marie-Tooth disease type 1X (CMT1X) is the second most common inherited peripheral neuropathy. Women with CMT1X typically have a less severe phenotype than men, perhaps because of X-inactivation patterns. Our objective was to determine the phenotype of women with CMT1X and whether X-inactivation patterns in white blood cells (WBCs) differ between females with CMT1X and controls. Thirty-one women with CMT1X were evaluated using the CMT neuropathy score (CMTNS) and the CMT symptom score in cross-sectional and longitudinal analyses. Lower scores correspond to less disability. WBCs were analyzed for X-inactivation pattern by androgen receptor X-inactivation assay in 14 patients and 23 controls. The 31 women's mean CMTNS was 8.35. Two-thirds of the cohort had a mild CMTNS (mean 4.85) and one-third had a moderate CMTNS (mean 14.73). Three patients had a CMTNS of 0. The pattern of X-inactivation did not differ between the affected and control groups. Women with CMT1X presented with variable impairment independent of age, type of mutation, or location of mutation. No evidence supported the presence of a gap junction beta-1 (GJB1) mutation affecting the pattern of X-inactivation in blood. Further studies are planned to determine whether X-inactivation is the mechanism for CMT1X females' variable phenotypes.     

A novel EGR2 mutation within a family with a mild demyelinating form of Charcot-Marie-Tooth disease

Mutations of the early growth response 2 (EGR2) gene have been reported in a variety of severe demyelinating neuropathies such as autosomal recessive congenital hypomyelinating neuropathy, autosomal dominant child-onset Dejerine-Sottas neuropathy, and autosomal dominant adult-onset Charcot-Marie-Tooth disease (CMT). Here, we report on a heterozygous mutation in EGR2 (c.1160C>A), which results in threonine at position 387 being changed to asparagine, in a family with a mild demyelinating form of adult-onset CMT. Of note, both the proband and her asymptomatic son exhibited neither pes cavus nor champagne-bottle leg atrophy, suggesting that the heterozygous T387N mutation may result in a relatively mild phenotype of demyelinating CMT.     

Outcome measures for Charcot-Marie-Tooth disease: clinical and neurofunctional assessment in children

Charcot-Marie-Tooth (CMT) disease is the most common inherited neuromuscular disorder, presenting with symptoms often occurring since childhood, and showing a progressive course. At present, there are no valid and reliable measures for evaluation of impairment and disability in the pediatric population. The aim of this study was to determine the usefulness of outcome measures, commonly used in adult patients, in CMT children. We report the results of a comprehensive evaluation of 21 children affected with CMT type 1A, including clinical examinations, measure of hand and foot muscle strength with a hand-held dynamometer, and the following scales: CMT Neuropathy Score or its clinical component CMT Examination Score, Overall Neuropathy Limitations Scale (ONLS), Walk-12 questionnaire, and nine-hole peg test (9-HPT). Hand grip, three-point pinch, and foot dorsiflexion strength were significantly lower than age/sex equivalent in almost all cases. 9-HPT was significantly abnormal in 62% of patients and CMT Examination Score was <10 points in all cases. ONLS showed presence of minor disability in the upper limbs in 57% and mild abnormalities of gait in 71% of patients. Overall, these scales demonstrated limited potential to measure disability and severity of the disease confirming that it is necessary to identify specific scales for children with CMT.     

MFN2‐related genetic and clinical features in a cohort of Chinese CMT2 patients

Charcot‐Marie‐Tooth disease 2A (CMT2A), caused by mutations in the mitofusin 2 gene (MFN2), is the most common CMT2 subtype. The aim of our study is to assess the frequency and summarize the genetic and clinical characteristics of Chinese CMT2A patients. A total of 17 coding exons of MFN2 were detected by direct sequencing in 82 unrelated Chinese families diagnosed as CMT2. Clinical evaluations were analyzed among CMT2A patients. We identified 14 missense variants in 9 sporadic and 6 familial cases, including four novel mutations (T129A, S249F, Q367P, and Q674L), 4 known mutations (R94W, R94Q, T105M, C132Y, M376V and Q751X), and 4 rare missense variants (K120E, C217F, K307E and T356S). A total of 23 patients had early‐onset phenotype. Two patients had a CMTNS score of 0 to 10; 16 had a score of 11 to 20; and 7 had a score greater than 20. Five patients were confirmed a de novo origin. Six of 14 variants were located or closed to the GTPase domain. We report four novel mutations and four rare missense variants. MFN2 mutations account for 18% of CMT2 families in mainland China. The common characteristics of Chinese pedigree are early disease onset and moderate phenotypes.     

A novel p.Glu175X premature stop mutation in the C-terminal end of HSP27 is a cause of CMT2

Mutations in the gene HSPB1, encoding the small heat shock protein 27 (HSP27), are a cause of distal hereditary motor neuropathy (dHMN) and axonal Charcot-Marie-Tooth disease (CMT2). dHMN and CMT2 are differentiated by the presence of a sensory neuropathy in the latter although in the case of HSPB1 this division is artificial as CMT2 secondary to HSPB1 mutations is predominantly a motor neuropathy with only minimal sensory involvement. A recent study in mice has suggested that mutations in the C-terminus result in a motor only phenotype resembling dHMN, whereas mutations at the N-terminus result in a CMT2-like phenotype. However, we present a family with a novel mutation in the C-terminus of HSP27 (p.Glu175X) with a motor predominant distal neuropathy but with definite sensory involvement compatible with CMT2. This case highlights the artificial distinction between patients with motor predominant forms of CMT2 and dHMN and argues against the hypothesis that mutations in the C-terminus have no sensory involvement.